In online giving, the acknowledgement emails go to the email address the donor indicated on the form, but the tax receipts go to the email address marked "primary" in the constituent record. This is confusing and inconsistent! Everything should go to the email address the donor used on the form.
This is planned as part of our upcoming "real time receipting" release where you can choose to have the pdf send as part of the donation confirmation email. Look for this release later this month!
Receipts sent manually or after the batch is approved will still send to the primary email address.
Do you think this will help with your scenario?